The present invention is directed generally to optical microscopy and in particular addresses the need to obtain efficiently, quantitative slope data from a differential interference contrast (DIC) microscope in two directions.
Many manufacturing processes require the measurement of microscopic surface topography. Applications include, but are not limited to, the control of surface finish in machined parts and the inspection of semiconductor wafers. Phase shifting interference, and more recently, white-light interference (WLI) microscopes are widely used to measure surface topography with lateral resolutions from less than one micron to tens of microns and height resolutions to less than a nanometer.
Phase shifting and WLI microscopes use optical interference between a reference surface and a sample to produce an interference image with fringes of constant height. Typical microscope types are based upon Mirau or Michelson interference objectives. A phase shifting interference microscope modulates the phase of the interference pattern by changing the relative length of the sample and reference arms of the interferometer. The acquisition and processing of a set of phase shifted interference images permits the calculation of the height of the sample at each pixel. The methods of phase shifting are well known.
WLI microscopes have superceded the use of phase shifting interference microscopes in almost all applications. One difference between a WLI microscope and a phase shifting microscope is the requirement for a short coherence length source in the former and its optional use in the latter. A major hardware difference between the two devices is the substantially larger vertical scan range of a WLI microscopexe2x80x94as much as hundreds of microns rather than less than one micron. The practical difference between the two types of microscopes is that a WLI microscope generates large amounts of data and uses substantially different processing algorithms to obtain a much greater measurement range than phase-shifting microscopes, with only a modest loss in precision. However, both microscopes share a significant problemxe2x80x94extreme sensitivity to environmental effects, especially vibration and air turbulence.
The environmental sensitivity of phase shifting and WLI interference microscopes is due to the non-common path nature of the test and reference arms of the instrument and the inherent sensitivity of optical interference. A Nomarski or differential interference contrast (DIC) microscope maintains the inherent measurement sensitivity of optical interference while removing the extreme environmental sensitivity because the two interfering light paths are nearly identical or common-path.
One of the earliest publications describing a DIC microscope is xe2x80x9cApplication à la mxc3xa9tallographie des mxc3xa9thodes interfxc3xa9rentielles à deux ondes polarisxc3xa9esxe2x80x9d, by G. Nomarski and A. R. Weill, Revue de Matallurgie, LII, #2, 1955, pp. 121-134. However, a DIC microscope is typically considered to be for qualitative use only. The reason for this is somewhat understandable. A WLI microscope produces fringes that are contour lines of constant height, just like a topographic mapxe2x80x94except the contour spacing is on the order of 0.3 microns rather than meters or more. A DIC microscope produces fringes that are contours of constant slope in one direction. There are two difficulties with using an interferometer that produces slope fringes. First, slope fringes are difficult to visually interpret and second, slope must be measured in two directions to fully reconstruct a surface.
When personal computers became widely available, the natural course of action was to automate the processing of what people were used to looking atxe2x80x94fringes of constant height. In modem manufacturing processes, visual inspection of images is avoided not only because it is slower but also because it is less reliable, precise and accurate than automated image processing. Modem computers are quite capable of performing the necessary computations on slope data from a DIC microscope to obtain surface topography data making it possible to take advantage of the environmental insensitivity of a DIC microscope.
In qualitative DIC microscopy, a rotating polarizer in conjunction with a quarter-wave plate is used to modify the image produced by the microscope so that features of interest are clearly visible. Rotating the polarizer or translating the DIC prism changes the relative phase of the two interfering beams providing the ability to phase shift (phase modulate) the interference image present on the detector. Devices based upon liquid-crystal technology may be used in place of the rotating polarizer and may optionally incorporate the quarter-wave plate as part of the liquid-crystal device. Multiple, phase shifted DIC images can be acquired and then processed using standard techniques from phase shifting interferometry to produce a quantitative measure of sample surface slope, where the slope measured is in the shear direction of the DIC prism. Quantitative DIC measurements were first presented in a paper by Hong et al in July 1993 at the annual SPIE conference; see, Gao Hong et al, xe2x80x9cThree-dimensional optical profiler using Nomarski interferometry,xe2x80x9d in SPIE, Vol. 1994, pp. 150-153, Advanced Optical Manufacturing and Testing IV, published February 1994, presented Jul. 11-Jul. 16, 1993 in San Diego, Calif. by Robert E. Parks (ISBN 0-8194-1243-0).
Delbert L. Lessor et al, in xe2x80x9cQuantitative surface topography determination by Nomarski reflection microscopy. 1. Theory,xe2x80x9d Journal of the Optical Society of America, Vol. 69, No.2, pp. 357-366 (February 1979) presented early theoretical work in 1979. Lessor et al; provide basic theory and propose rotating the sample to obtain slope data in two orthogonal directions; notably, this paper was not referenced by Hong et al.
A major limitation of a DIC microscope as compared to a WLI microscope is the need, in general, for rapid, robust measurement of slope in two directions. Another broadly applicable constraint is that a phase shifted interference and a DIC microscope are both limited to measurements within the depth of focus (DOF) of the objective, while WLI can go far beyond the DOF.
The present invention addresses how to obtain, rapidly and robustly, surface slope data in two shear directions through the use of wavelength multiplexing within a microscope. The discovery of how to use wavelength multiplexing to simultaneously obtain shear in two directions leads to several other related extensions of the technology. For example, wavelength multiplexing is accomplished in two somewhat different manners, one of which uses a xe2x80x9cdual field of viewxe2x80x9d (DFOV) optical system.
There are several other related approaches disclosed herein. One closely related approach is the sequential capture of slope data in two shear directions, resulting in a lower cost system.
The detailed discussion begins with a disclosure about the use of wavelength multiplexing to obtain slope data in two directions in a DIC microscope.
In accordance with the present invention, a differential interference contrast (DIC) microscope system is provided comprising:
(a) an illumination source for illuminating a sample
(b) a lens system for viewing the illuminated sample, including an objective, defining an optical axis;
(c) at least one detector system for receiving a sample image;
(d) mechanisms for wavelength multiplexing the shear direction or shear magnitude or both on the sample and demultiplexing the resultant DIC images on the detector; and
(e) a mechanism for modulating the phase of the interference image;
Various approaches are disclosed to accomplish wavelength multiplexing of shear direction and demultiplexing the two DIC images that result. It is possible for the two, wavelength multiplexed DIC images to differ in either or both shear direction or magnitude. These approaches include (1) two DIC microscopes, each operating at a different wavelength, but which share a single objective through a beam splitter; (2) a segmented DIC prism that is made in four sections where opposite sections are paired and have the same shear direction and amount, and each pair of sections have filters transmitting different wavelengths; (3) a segmented DIC prism that is located in or near an aperture stop or pupil of said DIC microscope to obtain data in two shear directions that is multiplexed by wavelength; (4) a dual field-of-view optical system with two DIC prisms, one in each path to wavelength multiplex shear direction or shear magnitude through said objective; (5) demultiplexing wavelength multiplexed DIC images through the use of a wavelength selective beam splitter and two detectors; (6) demultiplexing wavelength multiplexed DIC images through the use of a wavelength controlled source and a single detector; and (7) demultiplexing wavelength multiplexed DIC images through the use of dual field-of-view optics and a single detector.
The various approaches disclosed and claimed herein permit rapid, robust measurement of slope in two directions. A DIC microscope is insensitive to the environment, unlike other interference microscopes; however, slope data in two directions are necessary to determine, via integration, the surface shape, rather than just a height profile of the surface. The various approaches disclosed and claimed herein permit rapid, robust measurement of sample surface slope in two directions.
Further in accordance with the present invention, the differential interference contrast (DIC) microscope system alternatively comprises:
(a) the illumination source for illuminating the sample;
(b) the lens system for viewing the illuminated sample, including the objective, defining the optical axis;
(c) at least one detector system for receiving the sample image;
(d) the mechanism for modulating the phase of the DIC image; and
(e) a virtual reference surface stored in a computer provided with an image capture device, the virtual reference surface being a function of the focal position of the objective.
As used herein, a DIC prism shears (separates) one unpolarized ray into two orthogonally polarized rays. There is a direction and magnitude to that shear. At the detector, interference is observed occurs between two points on a sample that are separated by a distance and direction determined by the shear distance on the sample determined by the shear direction and magnitude in conjunction with the microscope focal length. A DIC microscope produces an image whose contrast uses interference to show differences in slope of the sample surface rather than height. Acquiring a set of phase modulated images wherein a rotating polarizer performs the phase modulation, and then processing the image data appropriately permits the measurement of sample surface slope in the shear direction with a sensitivity influenced by the shear magnitude. Integration of the slope data produces a height profile (using data in one shear direction) or height map (using data in two shear directions). Thus, the measured quantity is xe2x80x9cslope dataxe2x80x9d, which are measured in the xe2x80x9cshear directionxe2x80x9d. The shear direction is multiplexed via wavelength on the sample and the resulting DIC images are demultiplexed. Either or both shear direction and magnitude can be multiplexed.